Of course it is, and you are so welcome, hope I can help even a tiny little bit - you have my every sympathy here. Disciplinaries/grievances etc are my bread and butter, and I absolutely would not support any manager taking this shit forward! I really really hate to see people being badly treated!
So you haven't even had a formal confirmation that you are under investigation? That is so bad, especially as the world and its wife seems to be privy to this confidential information!!
They're on a hiding to nothing - an IT would tan their hide!!! - but it's still stressful to have to go through this, especially when you're not well in the first place.
Definitely make sure to state that their conduct has limited your access to representation. That is procedurally unfair before you even get started on the lack of confidentiality, the breach of trust and confidence (be a little careful with that one as they could potentially say that this means you can no longer work there), or the GDPR elements, not to mention the unreasonableness of an investigation in the first place, and the fact that other people have been informed of this before you have even had formal notice!! I am glad you have someone knowledgeable on your side - that means a lot. Your manager is a dick.
Yes, take notes by all means, and make sure they give you time to note everything that's said and that they don't rush you. If HR take notes, ask them to share those notes with you as well. Do feel free to ask for a break if you need one. A lot of people wouldn't think of that in advance. I would always 100% facilitate that and offer it as an option at the outset of any meeting. Also take water in case it's not provided, as you may need to drink some when you are talking a lot. IMO even when employees are clearly as guilty as hell of the allegations, they still should be treated with dignity and respect! I always take the view that making a mistake doesn't mean they are a bad person who deserves to be censured!
Much as I have had my profession run into the ground by certain elements here, I am always very much on the side of the employee unless there is good reason to the contrary. HR professionals are human too! We also go through the same shit as other employees. I recently had to get my union involved as I was being bullied by my line manager, and had to go LTS because it was having such a profound impact on my mental health.
Please do keep us updated as to how things go. Solidarity!!